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3.3 Approaches for Characterizing the Prevalence of Soil-Pica

Historical Document

This Web site is provided by the Agency for Toxic Substances and Disease Registry (ATSDR)
ONLY as an historical reference for the public health community. It is no longer being maintained and the data
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In response to the first charge question--"What observational, questionnaire, and analytical studies are most
valid for characterizing the extent of soil-pica behavior?"--the panelists discussed
the different methods that have been used to evaluate the prevalence of soil-pica
behavior, and identified strengths and weaknesses associated with each method.
Given the limitations of individual methods, the panelists eventually agreed
that a study that incorporates multiple methods would provide the best means
for validating the prevalence of soil-pica behavior and the distribution of
soil ingestion rates. The panelists thought that such validation was needed
to develop a robust characterization of the prevalence of soil-pica behavior.
Following is a summary of the panelists' discussion that led to this conclusion.

The utility of questionnaires.
Two panelists noted that surveys on soil ingestion, which researchers typically
administer to caretakers, are useful for getting meaningful insights on general
issues (e.g., "does your child ever eat dirt?") but are not particularly useful
for getting robust answers on specific issues (e.g., "how often does your
child eat dirt?" or "how much dirt does your child consume?") (SD, NF). Expanding
on this comment, one panelist indicated that parents typically do not observe
their children constantly and therefore are not able to comment reliably on
specific details of their soil ingestion behaviors (NF).

Other panelists listed reasons
why administering surveys to parents about their children's behavior might lead
to spurious results. First, parents might provide responses they think surveyors
want to hear, rather than responses that accurately reflect their children's
behavior (NF). Second, parents might provide inaccurate responses in efforts
to conceal information that might reflect badly on them as parents (e.g., their
children eating large amounts of dirt) (JM, DV). Third, because "unusual" behavior
is more easily recognized in families with multiple children, parents' perceptions
of "unusual" behavior might vary from one family to the next, thus complicating
efforts to characterize the prevalence of soil-pica behavior with surveys (PS).

The utility of analytical
studies. When discussing the different methods available to characterize
the prevalence of soil-pica, several panelists noted that soil ingestion rates
predicted by analytical methods (i.e., mass balance tracer research) have
varied considerably from one study to the next (SD, BL, DM). One panelist
suspected that these inconsistent findings might result from the difficulty
short-term analytical studies have identifying rare events (DM). Another panelist
agreed, explaining that the analytical studies he has conducted and reviewed
characterize soil ingestion behavior for a small number of people over a very
short time frame, typically 2 weeks or less (SD). With this study approach,
he thought the analytical studies have a very small chance of identifying
soil-pica events.

The panelists raised several other
concerns about analytical studies. For instance, one panelist was not convinced
that analytical studies can provide the most reliable account of soil ingestion,
given his experience conducting two studies, both of which found that soil ingestion
rates calculated from analytical approaches correlate very poorly with observational
accounts of mouthing behavior and soil ingestion (SD). Another panelist added
that inconsistent results might stem from the fact that studies are conducted
in different regions and among children of various socio-economic status (DM).
He was not surprised, for example, that soil ingestion rates observed among
children in suburban communities in Massachusetts were different from those
conducted on institutionalized children in Jamaica. One panelist did not think
the prevalence of soil-pica behavior among the Jamaican children should be viewed
as representative of that among children in the United States, given the small
size of the study and the living conditions of the children considered (BL).
Another panelist noted that mass balance studies assume an understanding of
the digestive processes and degree of uptake, transformation, and excretion
of tracers in young children (NF). She added that most tracer studies have been
conducted on children, who have different gut permeability, metabolism rates,
and excretion rates than adults. This reviewer commented that true mass balance
studies would collect both feces and urine over a long period of time, because
the temporal pattern of excretion in the two media will be different. She indicated
that the mass balance studies included in the review materials were based only
on fecal sampling over a limited time frame.

The utility of combining
several methods. Given their concerns about the various individual methods
for characterizing soil ingestion, most panelists advocated the use of multiple
methods in one study to derive a robust, validated distribution of soil ingestion
rates. Specifically, panelists highlighted the need for conducting an extensive
study that integrates information on levels of soil contamination, biomarkers
of exposure, and various metrics of soil ingestion (e.g., analytical, observational,
and surveyed accounts) (BL, RW). Several panelists noted that such a study
would be expensive, but thought the community at the Vasquez Boulevard/Interstate
70 Superfund site might be an excellent subject population for such research
(NF, BL). Though not disagreeing with these suggestions, one panelist cautioned
that using multiple methods to characterize soil ingestion has led to conflicting
results in his previous research and that using multiple methods to derive
a validated soil ingestion rate for pica children will be quite challenging,
though worth pursuing (SD).

When commenting on the use of integrated
approaches to characterize soil ingestion rates, two panelists identified existing
data sets that ATSDR should review. One panelist indicated that one of his past
studies on soil ingestion, which included observational and analytical components,
also has biomarker data (i.e., blood lead levels) that have yet to be thoroughly
examined (SD). Another panelist noted that biomarker data (i.e., levels of arsenic
in urine) are available for a Superfund site in Washington state where ingestion
of contaminated soils is of concern (JM).